The present invention relates to a method for straightening and stiffening starting sheets in electrolytic refining plants.
The invention also relates to an apparatus for carrying out the method according to the invention.
In the electrolytic refining of a metal, e.g. copper, the starting sheets detached from the base plate are attached to supporting bars and the starting sheets are then lowered into the electrolytic tanks, in which the starting sheets are allowed to grow into cathode plates.
In connection with their being detached from the base plate, the starting sheets often bend or even wrinkle. While being lowered into the electrolytic tanks or during other handling the starting sheets may be warped or their shape may otherwise change.
If the shape of the starting sheets deviates from a flat shape, the distances of the different points of the starting sheets from the anodes in the electrolytic tanks are not equal, and consequently those points of the starting sheets which are closest to the anodes grow fastest until they produce a short circuit.
In refining plants it has usually been necessary to move and adjust the starting sheets manually after they have been lowered into the electrolytic tanks, and in order to eliminate short circuits produced during the process it has been necessary to detach the starting sheets from the anodes by means of various auxiliary devices and to lift starting sheets from the electrolytic tanks and to straighten them by means of hand tools.
In order to reduce the above-described work stages, which require a great deal of manual labor and slow down the total process, and in order to improve the quality of the cathodes, the objective has been to straighten and stiffen the starting sheets to make them as flat as possible.
Starting sheets detached from the base plate have previously been straightened by means of rollers intended for the straightening of thin sheets and also by means of straightening devices specifically designed for the straightening of starting sheets. By means of several known devices, the unevenness of starting sheets can be substantially reduced and the shape obtained for the starting sheets is rather flat, but the stiffness of the starting sheets is not, however, sufficient. In order to prevent the starting sheets from changing their shape and in order to stiffen the starting sheets, fold and groove patterns have been made in them. The grooves or other protrusions deviating from the plane of the starting sheet surface must not be too deep or sharp-crested, since the anode slime and other impurities present in the electrolyte easily accumulate on the surface of starting sheets.
As regards the present state of the art, reference is made to U.S. Pat. No. 1,836,368, which discloses one example of the stiffening of starting sheets by means of a press and certain tools. By the method according to the invention, stiffening grooves covering nearly the entire surface of the starting sheets are obtained. However, stiffening of starting sheets by means of pressing has several disadvantages. Firstly, owing to the unevenness of the starting sheet surfaces and to the elasticity of the material, several starting sheets are considerably warped during the stiffening. Secondly, the tools required for the stiffening are quite expensive. Thirdly, the stiffness obtained is often insufficient, and starting sheets still become distorted during handling.
Starting sheets have also been stiffened by the rolling of grooves or other stiffening patterns into them, but the problems are similar to those encountered in stiffening by means of a press. Owing to variation in the starting sheet thickness and to variation in their crystal structure, it is not possible to control the rolling procedure, and the starting sheets are warped during the rolling and, furthermore, the stiffness produced is usually insufficient.
As regards the present state of the art, reference is made to U.S. Pat. No. 3,544,431. In the method according to this patent, the starting sheets detached from the base plate are straightened and stiffened by rolling, whereafter the starting sheets are allowed to grow in the electrolytic tanks to about double their thickness. Thereafter the starting sheets are lifted out of the electrolytic tanks and transferred to a press, by which the starting sheets are re-straightened and re-stiffened. Thereafter, the starting sheets are returned to the electrolytic tanks to grow into the final cathodes. This prior known method includes a very complicated, operationally vulnerable and expensive press unit. In addition, lifting the starting sheets out of the electrolytic tanks in between and transferring the starting sheets to the press and then back into the electrolytic tanks require exceptional arrangements in the refining plants, regarding the use of space, for example. The lifting capacity is also taken to its limit in lifting the starting sheets back and forth. It is not possible to treat an entire electrolytic tank full of starting sheets, but depending on the size of the electrolytic tank the starting sheets can be treated in two or several batches. Furthermore, the acquisition costs of the equipment required for this known method are high.
The object of the present invention is to provide an improvement to prior known methods for the straightened and stiffening of starting sheets used in electrolytic refining plants. The object of the invention is to provide a method which makes it possible to obtain sufficiently flat and stiff starting sheets, whereby the number of short circuits produced between the starting sheets and the anodes in the electrolytic tanks can be reduced substantially. A more specific object of the invention is to provide a straightening and stiffening method for starting sheets, a method in which variations in the starting sheet thickness, in the evenness of the surfaces and in the properties of the material do not cause warping of the starting sheets or any other undesirable phenomena. The other objectives of the invention and the advantages gained using it are disclosed in the description of the invention.